EP0127301B1

EP0127301B1 - Improved positive battery plate

Info

Publication number: EP0127301B1
Application number: EP84302574A
Authority: EP
Inventors: John J. Rowlette
Original assignee: California Institute of Technology CalTech
Current assignee: California Institute of Technology CalTech
Priority date: 1983-04-25
Filing date: 1984-04-16
Publication date: 1989-10-04
Also published as: US4507372A; DE3480040D1; JPS601758A; EP0127301A1

Description

Background Art

Even though there has been considerable study of alternative electrochemical systems, the lead-acid battery is still the battery-of-choice for general purpose uses such as starting a vehicle, boat or airplane engine, emergency lighting, electric vehicle motive power, energy buffer storage for solar-electric energy, and field hardware whether industrial or military. These batteries may be periodically charged from a generator.
The conventional lead-acid battery is a multi- cell structure. Each cell contains a plurality of vertical positive and negative plates formed of lead-based alloy grids containing layers of electrochemically active pastes. The paste on the positive plate when charged contains lead dioxide which is the positive active material and the negative plates contain a negative active material such as sponge lead. This battery has been widely used in the automotive industry for many years and there is substantial experience and tooling in place for manufacturing this battery and its components. and the battery is based on readily available materials, is inexpensive to manufacture and is widely accepted by consumers.
However, during discharge, the lead dioxide (a fairly good conductor) in the positive plate is converted to lead sulfate, an insulator. The lead sulfate can form an impervious layer encapsulating the lead dioxide particles which limits the utilization to less than 50% of capacity, typically around 30%. The power output is significantly influenced by the. state-of-discharge of the battery, since the lead sulfate provides a circuit resistance whenever the battery is under load. Furthermore, the lead sulfate can grow into large, hard, angular crystals, disrupting the layer of paste on the grid resulting in flaking and shedding of active material from the grid. Power consumption during charge is also increased due to the presence of the lead sulfate insulator. The lead sulfate crystals in the negative electrode can grow to a large, hard condition and, due to their insulating characteristics, are difficult to reduce to lead. Even when very thin pastes are utilized, the coating of insulating lead sulfate interferes with power output. Thus, power capability is greatly influenced by the state-of-charge of the battery.
An apparent solution to this problem would be the addition of a conductive filler to the paste. The filler must be thermodynamically stable to the electrochemical environment of the cell, both with respect to oxidation and reduction at the potential experienced during charge and discharge of the cell, and to attack by the acid.
It has been attempted to increase the conductivity of the paste by adding a conductive filler such as graphite. Graphite has been used successfully as a conductive filler in other electrochemical cells, such as in the manganese dioxide positive active paste of the common carbon-zinc cell, and mixed with the sulfur in sodium-sulfur cells. However, even though graphite is usually a fairly inert material, it is oxidized in the aggressive electrochemical environment of the lead-acid cell to acetic acid. The acetate ions combine with the lead ion to form lead acetate, a weak salt readily soluble in the sulfuric acid electrolyte. This reaction depletes the active material from the paste and ties up the lead as a salt which does not contribute to production or storage of electricity. Highly conductive metals such as copper or silver are not capable of withstanding the high potential and strong acid environment present at the positive plate of a lead-acid battery. A few electrochemically-inert metals such as platinum are reasonably stable. But the scarcity and high cost of such metals prevents their use in high volume commercial applications such as the lead-acid battery. Platinum would be a poor choice even if it could be afforded, because of its low gassing over-potentials.

Statement of the Invention

An improved lead-acid battery is provided in accordance with the invention in which the positive active material maintains conductivity during both charge and discharge cycles. The power output in the battery of the invention is more uniform since it is less dependent of the state-of-charge of the battery and more nearly approaches theoretical efficiency.
The improved power characteristics are provided by incorporating a material into the paste that is insoluble in the electrolyte, has a conductivity similar to the active material and is thermodynamically stable with respect to oxidation and reduction when it is subjected to the usual charging and discharging potentials of a lead-acid battery.
According to the present invention there is provided a positive paste for a lead-acid battery comprising a layer comprising a matrix of lead oxide containing a dispersion of solid tin oxide in an amount sufficient to provide conductivity to the layer when charged or discharged, characterised in that said tin oxide is coated onto a particulate substrate.
Also in accordance with the present invention there is provided a lead-acid battery cell comprising in combination: an electrolyte impervious enclosure for receiving a body of liquid acid electrolyte; a first electrode containing negative active lead material immersed in the body of electrolyte; a second electrode immersed in said body of electrolyte, said second electrode containing a positive paste in accordance with the invention; a positive and negative terminal; means connecting the electrodes to their respective terminals; and means connected to the electrodes for preventing tin oxide reduction.
A suitable tin oxide additive for the positive active paste in accordance with the present invention is tin dioxide (Sn0₂) which can be predispersed in the paste or added in precursor form. Sn0₂ can be present asa-powde.corcoated onto a particulate or fibrous substrate such as glass powder or glass wool. Stannic oxide has a conductivity several times that of graphite. Sn0₂ (doped) has a conductivity of 300 to 400 micro ohm cm vs. 1375 micro ohm cm for graphite.
Stannic oxide is thermodynamically stable to the oxidation/reduction potential in the electrochemical environment of a lead-acid battery, has about the same resistivity as Pb0₂ when Sn0₂ is coated onto glass, and refractory or baked type of Sn0₂ is quite insoluble in sulfuric acid. Unlike Pb0₂, the stannic acid conductivity additive will remain unchanged during the course of charge and discharge of the positive plate.
These and many other features and attendant advantages of the invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.

Brief Description of the Drawings

Figure 1 is a graph of the open circuit voltage (OCV) of a lead acid cell and of each electrode and of the potential of each electrochemical reaction;
Figure 2 is a schematic view of a lead-acid cell;
Figure 3 is a sectional view of a positive plate containing conductive filler taken along line 3-3 of Figure 2;
Figure 4 is a schematic view of a positive plate containing a positive active layer in which positive active materials are dispersed in a conductive filamentary wool.

Detailed Description of the Drawings

Referring now to Figure 1, the electrode potentials referred to the standard hydrogen electrode are shown for the open circuit voltages for the cell and of each plate. Also shown are the potentials of various reactions occurring in a lead-acid battery. The potential region below which Sn0₂ is reduced is shown in the cross-hatched region. A schematic cell is shown in Figure 2. The cell 10 is housed in an electrolyte-impervious container 12 forming a cell compartment 14 in which is received a positive plate 16, negative plate 18 and a body 20 of aqueous sulfuric acid electrolyte. The cell can be connected in parallel or series to other cells, not shown. The negative plate 18 and positive plate 16 are connected to terminals 22, 24.
Positive plate reversal to less than about -0.5 V vs. SCE is to be avoided to prevent reduction of the Sn0₂ to tin. This can be accomplished by utilizing an over-sized positive plate and placing a 5 to 10% precharge in the positive plate so that even when discharged, there is no danger of falling into the reduction zone for stannic oxide. Another means of preventing cell reversal is to place a circuit breaker 26 in series with the plates 16, 18 that prevent passage of current between the plates when the positive electrode voltage is below about -0.5 V vs. SCE.
The negative plate 18 is of standard construction and is formed of a high area conductive substrate such as a lead on antimony-lead alloy ladder grid on which is deposited a layer of negative active material such as sponge lead. The negative active material can be formed by reducing a paste of lead oxide or tetra-basic lead sulfate to sponge lead.
Referring now to Figure 3 the positive plate 16 can also be formed of a conductive ladder grid 30 containing a layer 32 of lead dioxide in which is dispersed 1 to 10% by weight of conductive tin oxide in particulate form such as random fibers 34. The fibers form a conduction path through the layer 32.
The tin oxide is coated onto a substrate such as glass in powder or fiber form. Glass wool can be utilized to form the continuous phase for the layer of paste. As shown in Figure 4 the positive active layer 40 on grid 44 comprises glass wool 42 containing a conductive coating 46 of stannic oxide. The glass wool is impregnated with a paste 48 of lead dioxide and dried to form the layer 40 which contains a continuous conduction path through the filamentary glass wool from the outside surface 50 facing the electrolyte to the back surface 52 in contact with the grid 44. The stannic oxide coated glass wool can be chopped into roving or short lengths of glass fiber, or powder can be coated with conductive stannic oxide and dispersed in thewet paste before application to the grid.
The coating of stannic oxide onto glass to form conductive coating was developed over 30 years ago and has been widely practiced to defrost windshields in aircraft and automobiles. The conductive coating is applied to heated glass fibers or powder or glass wool from a solution of stannic chloride in hydrochloric acid as disclosed in U.S. Patent No. 2,564,707, the disclosure of which is expressly incorporated herein by reference. The solution can be sprayed onto the heated fibers or the fibers can be pulled through a tank containing the solution and then heated in a furnace to a suitable high temperature such as about 700°C.
The diameter of the glass fibers is preferably very small such as from 1 to 20 microns (¡lm). Very fine fibers are too hard to handle and large diameter fibers have too small a surface to provide adequate conductive surface. The fibers contain a conductive coating of stannic oxide from a monolayer in thickness to 10 microns (pm), usually from 0.01 micron (um) to 5 microns (jim).

Claims

1. A positive paste for a lead-acid battery comprising:

a layer comprising a matrix of lead oxide containing a dispersion of solid tin oxide in an amount sufficient to provide conductivity to the layer when charged or discharged, characterised in that said tin oxide is coated onto a particulate substrate.

2. A paste according to claim 1, characterised in that the substrate is in powder, filamentary, or fiber form.

3. A paste according to claim 2, characterised in that the substrate is a glass fiber having a diameter of from 1 to 20 um.

4. A paste according to claim 3, characterised in that the tin oxide is present as a coating having a thickness from a monolayer to 10 µm.

5. A paste according to any preceding claim, characterised in that the coated fibers are present in the paste in an amount of from 1 to 10% by weight.

6. A lead-acid battery cell comprising in combination:

an electrolyte impervious enclosure for receiving a body of liquid acid electrolyte;

a first electrode containing negative active lead material immersed in the body of electrolyte;

a second electrode immersed in said body of electrolyte, said second electrode containing a positive paste as claimed in any one of claims 1 to 5;

a positive and negative terminal;

means connecting the electrodes to their respective terminals; and

means connected to the electrodes for preventing tin oxide reduction.

7. A battery cell according to claim 6, characterised in that an oversized positive electrode is utilised which has a 5-10% precharge.

8. A battery cell according to claim 6 or 7, characterised in that a circuit breaker is placed in series with the electrodes.